The embryonic chick cardiovascular system will be used to substantiate the possible relationship between hemodynamic alteration and the production of cardiovascular malformations. Catecholamines and methylxanthines, compounds which have been shown to induce cardiac and aortic arch anomalies in the embryonic chick (Teratology 12: 33-46, 1975; Circulation 54, Supplement 2: 185, 1976) will be administered to embryos at stages of embryogenesis which have proven quite sensitive to these agents. Microcinematography, time-lapse microphotography, heart rate and contracted state determinations, histology, electron microscopy and blood pressure measurements shall be employed in search for valuable clues to one or more mechanisms involved in abnormal cardiogenesis and aortic arch development.